Acrylonitrile fibers have hitherto been used broadly in the field of clothing because of their characteristics, such as excellent light resistance, dyeing properties, and the like, but have scarcely been utilized for industrial use due to inferior mechanical strength as compared with nylon and polyester fibers. Development of high-strength acrylonitrile fibers has, therefore, been keenly desired.
In general, strength of acrylonitrile fibers decisively depends on properties of the polymer used. For example, Japanese Patent Application (OPI) No. 21905/85 (the term "OPI" as used herein means an "unexamined published patent application") discloses that an acrylonitrile polymer having a high average molecular weight, i.e., a reduced viscosity of 2.6 or higher, produces fibers having increased strength. However, the spinning solution according to this disclosure has an extremely high viscosity as shown in the following table indicating the relationship between the polymer concentration and the viscosity of the spinning solution at 45.degree. C. In particular, when the polymer having high molecular-weight is used in the spinning solution in order to improve the fiber property, the viscosity of spinning solution becomes extremely high, and such a spinning solution is inferior in extrudability from a spinning nozzle, coagulation characteristics, and stretching characteristics of coagulated filaments.
______________________________________ Polymer Concen- 6 8 10 12 tration (wt %) Viscosity of 1300 2100 5200 9000 Spinning Solution (poise) ______________________________________
The acrylonitrile polymer in such a high viscous spinning solution having a viscosity exceeding 1300 poises, and particularly exceeding 2000 poises, as measured at 45.degree. C., are considerably restricted in their use. In other words, electron microscopic observation of coagulated filaments obtained by extruding a high viscous spinning solution into the coagulating bath reveals that the acrylonitrile polymer is not oriented in the direction of a fiber axis, but rather shows three-dimensional irregularity. It is very difficult to re-orient the acrylonitrile polymer of the coagulated fiber by subsequent wet heat stretching or dry heat stretching. As a method to reduce the viscosity of spinning solution, the method in which the polymer concentration in the spinning solution is decreased is generally used. However, when the spinning solution having low polymer concentration is spun, dissolution is caused in a large amount, and therefore the fiber structure is liable to be porous and to form devitrification, and fusion of coagulated filaments is liable to occur. Thus, methods to obtain a spinning solution sufficient in the fiber-forming properties by using high molecular-weight polymer without increasing the viscosity of spinning solution are still desired.